1. Field of the Invention
The present invention relates to a method and apparatus for processing a target substrate having a silicon layer on the surface, such as a silicon wafer, to oxidize the silicon layer and form a silicon oxide film.
2. Description of the Related Art
Films of various kinds used in semiconductor devices need to be made thinner, owing to the demands of multi-layered and thinner arrangement of semiconductor devices used as components in semiconductor integrated circuits. For example, as regards the gate insulating film of MOSFETs, a method has been studied of first forming a silicon oxide film having a very small thickness of about 1 nm, and then irradiating the silicon oxide film with, e.g., nitrogen plasma for nitridation to form an insulating film. In order to form such very thin silicon oxide films with high planar uniformity, high inter-surface uniformity (among target substrates), and high reproducibility, the oxidation process should be performed at a very low oxidation rate (film-formation rate).
On the other hand, as the process temperature is lowered to reduce the oxidation rate, the film quality tends to be more deteriorated. Accordingly, it is preferable that the process temperature is not lowered so much, but is set to be, e.g., 650° C. or more. As an oxidation method, there is dry oxidation, which employs oxygen gas, and wet oxidation, which employs water vapor. An oxide film formed by wet oxidation is higher in film quality than an oxide film formed by dry oxidation. However, where a reaction tube is supplied with water vapor generated by a combustion method of burning hydrogen and oxygen, it is difficult to form very thin oxide films with good controllability, and thus is difficult to form oxide films with high uniformity and high reproducibility. This is so, because, in this case, an oxide film is formed at a high film-formation rate due to a high ratio of water vapor relative to a mixture gas of water vapor and oxygen.
Jpn. Pat. Appln. KOKAI Publication No. 11-204511 (see FIG. 1, and paragraph numbers [0020] and [0022]) discloses an oxidation apparatus including a water vapor generator, which generates a small amount of water vapor by causing hydrogen and oxygen to react with each other in the presence of a catalyst. FIG. 8 is a side view showing a conventional vertical heat-processing apparatus of this type.
As shown in FIG. 8, this apparatus includes a vertical reaction tube (reaction container or process chamber) 11, and a load-lock chamber 12 defining a loading area therebelow. A water vapor generator 14 is connected to the reaction tube 11. The generator 14 generates water vapor by causing hydrogen and oxygen to react with each other in the presence of a catalyst, such as platinum or nickel. A wafer boat 13 is employed to hold a number of semiconductor wafers W, in a state where they are stacked at intervals. When a process is performed, the reaction tube 11 and loadlock chamber 12 are vacuum-exhausted to a predetermined vacuum level. Then, the wafer boat 13 is loaded into the reaction tube 11. Then, water vapor is supplied into the reaction tube 11 to form a silicon oxide film on the wafers W.
International Publication No. WO 98-39802 (see page 43, lines 4 to 24, claim 8, and FIGS. 9, 11, and 13) discloses a so-called process apparatus of the cluster tool type, which includes an airtight transfer chamber and a plurality of single-wafer process chambers connected thereto. This apparatus also includes a water vapor generator, which generates water vapor by causing hydrogen gas and oxygen gas to react with each other in the presence of a catalyst. When a process is performed, a susceptor disposed in a process chamber is first heated by a heater. At this time, it is set such that a wafer is to be heated to a process temperature of 800 to 900° C. when the wafer is placed on the susceptor. Then, the process chamber is purged by nitrogen gas, and a wafer is loaded into the process chamber. Then, moisture and non-reacted oxygen gas are supplied from the water vapor generator into the closed process chamber, to oxidize the wafer surface and form a gate insulating film having a thickness of 5 nm or less.
Jpn. Pat. Appln. KOKAI Publication No. 11-186255 (see FIG. 3, and paragraph numbers [0038] and [0059]) discloses another method of forming a silicon oxide film. According to this method, a first film-formation step is performed at a lower temperature to form a silicon oxide film on the surface of a silicon layer, wherein the silicon oxide film functions as a protection film as well. Then, a second film-formation step is performed at a higher temperature to reduce the surface roughness of the silicon layer. In this case, a heating step is performed in an inactive gas atmosphere, pressure-reduced atmosphere, or oxidation atmosphere containing a wet gas.